Hello. I'm a network engineering student with zero experience. Inspired by the recent flood and power outage problem.I'm doing a paperwork on zero system downtime for business continuity . SAN is out of the question due to it's cost.

My scenario: 3 branch located at different places with one of them hosting the primary server and another one of them hosting the secondary server.

From my theoretical knowledge. I'm suppose to connect these branches with a gateway on each with a permanent public IP. I'm guessing ISDN. So that's my connection. I think that the primary and secondary servers should be sychronised.

Suppose the primary server isn't accesible, how do I make sure the secondary server would take it's place immediately with zero downtime. I heard about failover DNS.

1. What is the best way I could do to connect these 3 branches with security in mind ?
2. What is the best method I can do to synchronize or mirror the primary and secondary server?
3. Everything else I should know

Sorry for my complicated problem and Thank you very much in advance.